Alkyl esters of fatty acids are commonly used as biodiesel fuel, and are a beneficial alternative fuel source compared to conventional fossil fuel sources. Fatty acid alkyl esters are produced by a transesterification reaction between triglycerides (animal fat and/or vegetable oil) and alcohol to form product esters and glycerol.
Transesterification processes for producing biodiesel alkyl esters are well known in the art. Historically, triglycerides in fats and oils have been methylated or otherwise esterified in a two-step process using an acidic catalyst, such as is described in the U.S. Pat. No. 4,695,411 to Stern et al., U.S. Pat. No. 4,698,186 to Jeromin et al., and U.S. Pat. No. 4,164,506 to Kawahara et al. Such processes included a pre-transesterification step, where fats/oils were reacted with alcohol in the presence of an acidic catalyst, and a subsequent transesterification step, where an alkaline/alcohol blend was added for the reaction completion. Transeterification processes employing alkaline catalysts, such as U.S. Pat. No. 5,525,126 to Basu et al., U.S. Pat. No. 5,908,946 to Stern et al., and U.S. Pat. No. 6,538,146 to Turck et al., are known in the art, as well.
The transesterification reaction is usually carried out in a reaction medium having a stoichiometric excess of alcohol, because this accelerates the production of useful esters. The excess unreacted alcohol present in the product mixture is distributed in both the ester rich light phase product and the glycerol rich heavy phase product. The excess alcohol in the product streams needs to be recovered so that the alcohol can be recycled to the process as a reactant, and to enhance the purity of the ester and glycerol product streams. Various methods of purifying the product streams and recovering excess alcohol have been described in the art.
U.S. Pat. No. 5,424,467 to Bam et al. describes a purification process in the manufacture of biodiesel esters whereby the unreacted alcohol in the glycerol rich product stream is removed by extractive distillation, and then the purified glycerol stream is contacted with the ester product stream in a liquid-liquid extraction unit to purify the ester product and to extract unreacted alcohol.
United States Patent Application Publication No. 2006/0244005 to Felly describes a process for making biodiesel where the product stream is cooled and allowed to separate into an ester rich phase and a glycerol rich phase. Alcohol vapor is recovered from the glycerol rich phase by boiling it under vacuum conditions by applying a vacuum pump. The alcohol is removed from the ester rich phase by washing with water. Additionally, alcohol vapor is recovered from the reactor and various holding tanks by applying a vacuum to the top of each unit. The vapor from the tanks and from the glycerol boiler is then sent to a condensing unit to recover the alcohol.
U.S. Pat. No. 7,126,032 to Aiken describes a process for purifying the glycerol rich product stream in biodiesel ester production, whereby the glycerol rich stream, which contains minor portions of alcohol and methyl ester, is heated to produce additional amounts of alcohol and glyceride. The stream is then sparged with nitrogen which strips the unreacted alcohol out of the stream. The unreacted alcohol vapor is subsequently condensed and collected.
U.S. Pat. No. 7,045,100 to Ergün et al. describes a biodiesel production system where the methyl ester product may be separated from the glycerol product by a filtration apparatus.
U.S. Pat. No. 6,174,501 to Noureddini relates to a process for making biodiesel esters where the glycerol rich product is decanted from the methyl ester rich product stream. The glycerol rich stream, which is at low pressure, is then sent through a deionizer to remove cations, and then sent to a flashing unit to remove methanol. The glycerol stream subsequently undergoes additional purification.
Other approaches for recovering excess alcohol have been described in the art, including a process where the ester rich phase is washed with water to extract the unreacted methanol, the water wash is combined with the glycerol rich phase, and then the methanol is distilled off from the combined glycerol/wash stream. See, English et al., Economic Feasibility of Producing Biodiesel in Tennessee, p. 50, published by Agri-Industry Modeling & Analysis Group (December 2002).
While the above described approaches facilitate the recovery of unreacted alcohol, they require substantial additional equipment and excessive energy expenditure. For example, the Felly reference described above uses numerous holding and decanting tanks, as well as expensive vacuum equipment to extract the unreacted alcohol. There accordingly exists a need for an efficient, cost-effective transesterification process whereby the unreacted alcohol component can be readily separated from the product ester or product glycerol streams, and be reused as a reactant component.